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Beomaster 1900 amplifier issues

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bob
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bob posted on Fri, Jun 7 2019 11:43 PM

Guys

working on my beomaster 1900 2904. 

 One channel the left works fine no problems at all right channel will throw it into protection mode and into standby instantly.

 Pull the output transistors from the right channel unit comes on plays one side only seems to work fine .

 The output transistors test OK I don’t see anything burn no resistors immature resistors no damage .

 What I do see on the base of the output transistors is like 30 V ,

 My first assumption was OK I have a shorted transistor in the chain ,  replaced every single transistor 307 on out to the drivers all tested fine ,  but when I fired up with 30 V on the base for the outputs is still there .  This guy has been fully recapped also . Transistors 307-310 in,  no crazy high voltage at the base terminal .

Visually like I say I don’t see any trays damage reheated a bunch of solder joints on the amplifier board double checked and triple checked for any burnt components checked a few resistors I’m just not seeing anything. 

 Seems to begin with transistor 311 on,  what’s weird is I D Sauter transistors from the finals going back I can aluminate that high voltage so it’s something common to the biasing.  I’m studying the schematic so it Hass to be a resistor capacitor that is common to both the end and the pea sides of the amplifier.  

Any thoughts I’d appreciate it thanks

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jesperb
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Bit late with the answer, but i had the same issue on my 2400.
Standby when powered on. Pulled left outout tranny.
Startup now OK and played fine in the right channel only.
Had recapped and swapped all trannies in the output stage left channel, except for the output tranny as it measured out fine.

Swapped the left output tranny for a new (about €2 apiece) and everything was OK.

Just my €5

 

Beogram TX, Beovox S45, Beovox MC 120.2, BeoSound 1, CX50, Beovox S75, Beomaster 2400

bob
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dayton, Ohio
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bob replied on Tue, Jul 2 2019 11:01 PM

 I finally did get it worked out what concerned me was without the output transistors fairly high voltage going to their base connection . 

I was concerned with that checked and rechecked every transistor and amplifier the voltage is just weren’t coming out right. 

Everything was fine right up to TR 312, After that all over the map, Reading up on base current I realize there must be relatively healthy  draw At the base connection of the outputs. 

Put in a fresh set of transistors for the outputs everything stabilized. 

 You live you learn I was being very cautious didn’t want to take a chance on destroying the transistors that I just put back in,  so after a week of studying measuring and looking at the DC path I decided to give it a shot. 

Amplifier measures out fine motherboard measures out fine but I’ve got a channel level imbalance in the tone/preset board.

 It’s fine coming from the motherboard fine coming out of the balance controls and then off feeding into the power amp section,  go to trace the signal path with an oscilloscope Next.

 Very nice receiver get bedbug worked out it’ll be fine.

 

jesperb
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Like you write yourself 'You live and you learn...'. I also took quite a long break, before swapping the output transistor - and for the exact same reasons ;-)

Re. the tone level, I recapped the tone/balance board completely - not only electrolytics.
Of course with caps too tall, to fit under the tone control panel itselt.
However, replacing the MKT (polyester ?) condensators, did remove 'muddy' sound in one channel. Didn't measure it out with the scope.

Not sure if your problem can be related to the no-signal current ?
Did you replace the two potentiometers for no-signal current adjustment ? If they short out, the can kill your output stage (again).
Did you re-adjust no-signal current after swapping them out ? 

Beogram TX, Beovox S45, Beovox MC 120.2, BeoSound 1, CX50, Beovox S75, Beomaster 2400

bob
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bob replied on Wed, Jul 3 2019 11:07 AM

Oh yeah the output current absolutely. adjusted that immediately upon firing it up I had meters attach to see if there was any excessive current.

 

bob
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bob replied on Wed, Jul 3 2019 11:07 AM

Oh yeah the output current absolutely. adjusted that immediately upon firing it up I had meters attach to see if there was any excessive current.

 

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